By Christopher Schwarz
Woodworking Magazine, September 2005, pages 12-15
Drawboring is one of the simple reasons that so much antique furniture survives today, some of it as sound as the day it was made.
What is drawboring? It’s a technique that greatly strengthens a mortise-and-tenon joint, transforming it from a joint that relies on glue adhesion into a joint that has a permanent and mechanical interlock. In essence, you bore a hole through both walls of your mortise. Then you bore a separate hole through the tenon, but this hole is closer to the shoulder of the tenon. Then you assemble the joint and drive a stout peg through the offset holes. The peg draws the joint tight.
Drawboring offers several advantages compared to a standard glued mortise and tenon:
■ The joint will remain tight. A common problem with mortise-and-tenon joints is that the joint can open up and develop an ugly gap at the shoulder. Sometimes this is caused by the wood shrinking as it reaches equilibrium with a new environment (such as your living room with its forced-air heat). Sometimes this gap is caused by simple seasonal expansion and contraction, especially with woods that tend to move a lot, such as flat-sawn oak. The peg in a drawbored joint keeps the tenon in tension against the mortise during almost any shrinkage.
■ The joint can be assembled without clamps. Drawboring is excellent for unusual clamping situations. Driving the peg through the joint closes it and clamps are generally not needed. Chairmakers use drawboring to join odd-shaped pieces at odd angles. It’s also an excellent technique when your clamps aren’t long enough. Or when you don’t have enough clamps. Drawboring also allows you to assemble a project one piece at a time if need be.
■ The joint can be assembled without glue. There is good evidence that drawboring allowed early joiners to assemble their wares without any glue. This is handy today when you’re joining resinous woods (such as teak) that resist modern glues or when you’re assembling joints that will be exposed to the weather, which will allow water to get into them and destroy the adhesive.
■ The joint doesn’t have to be perfect. The mechanical interlock of drawboring means that your tenon’s cheeks don’t have to have a piston fit with your mortise’s walls. In fact, you might be surprised at how sloppy the joint can be and still be tight after hundreds of years. Drawboring requires you to be careful only when fitting the tenon’s shoulder against your mortised piece. The other parts of the joint are not as important. And while I never argue against doing a good job, drawboring ensures that every joint (even the less-than-perfect ones) can be tight for many lifetimes. For this reason, I think drawboring is an excellent basic skill for beginning woodworkers.
So why has drawboring become an almost-lost art? It’s a good question, and one that I cannot fully answer. I suspect that modern glues and machine-made joinery made the technique less necessary, particularly for manufactured furniture. Drawboring does require several extra steps, and the benefits of it – particularly the long-term durability of the joint – is not something that is apparent to a customer.
Another reason the technique has fallen out of favor, I suspect, is that manufacturers have stopped making drawbore pins. These tapered steel tools allow you to temporarily assemble the joint to check the fit and to ease the path that the wooden peg will later follow. You can drawbore without drawbore pins by relying on the peg (and luck) alone. But once you use a proper set of drawbore pins, you will wonder why they are not in every tool catalog. Fortunately, you can make your own set of drawbore pins inexpensively. The story starting on page 14 shows you how.
I have drawbored many joints during the last five years or so and have found the methods described here to be highly effective. My method is based on historical descriptions of the process from the 17th century and my own work.
The first detail to tend to is the size and location of the hole through the mortise. I have found that a 1⁄4″-diameter hole is good for cabinet work. For larger-scale work (workbenches, doors and windows for homes) a 3⁄8″-diameter hole is better because the peg is stouter. In general, place the hole 3⁄8″ from the opening of the mortise in furniture work and 1⁄2″ in larger work. Make the hole as deep as you can. Usually this requires boring it through the entire assembly, though the hole can be stopped in thick stock. The goal is to ensure that the untapered part of the peg passes into the other wall of the mortise.
Historically, many of the drawbore pins I’ve encountered are a diameter that’s best suited for a 3⁄8″-diameter hole and peg. Entryway doors and large windows are appropriate for this larger hole and peg. I have encountered (and own) a set of old pins that work with a 1⁄4″-diameter hole, however, so this approach is historically accurate.
The next thing to consider is how much to offset the hole in the tenon. The bigger the offset, the sounder the joint, but the bigger the risk that you’ll destroy the tenon or peg during assembly.
The traditional joiner was advised to offset the holes by the width of a shilling, according to Joseph Moxon’s “Mechanick Exercises,” a 17th-century how-to book on woodworking. I had difficulty locating a shilling from the middle to late 17th century (I did try), but according to one knowledgeable collector of English coins, a 17th-century shilling would be about 1⁄16″ thick.
An offset of 1⁄16″ will indeed almost always work and is easy to assemble. But I’ve found that it’s sometimes not enough to get the job done. Some of the joints I assembled with this small offset were just a bit wiggly. For furniture-scale work, I prefer a 3⁄32″ offset. For big-scale work, I’ll push that offset to almost 1⁄8″ if the parts of the joint are large and the wood is a tough species, such as ash or elm. Experience will be your guide. Begin with small offsets in a sample joint and gradually increase them. You’ll know when you’ve found the sweet spot.